parallel_emerge: Show build target based on sysroot, not board.

It's the sysroot value that is the authoritative indication of what
we're building for / installing to. Therefore, print that instead of the
board value (which might not be set).

BUG=brillo:918
TEST=parallel_emerge --sysroot=/build/link prints 'Building package ...
on /build/link' as expected

Change-Id: I19b982c9637a776f57bb02569e9471e9d2c37248
Reviewed-on: https://chromium-review.googlesource.com/272940
Trybot-Ready: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/scripts/parallel_emerge.py b/scripts/parallel_emerge.py
index 8910a2a..a5da2c7 100644
--- a/scripts/parallel_emerge.py
+++ b/scripts/parallel_emerge.py
@@ -1851,7 +1851,7 @@
     cmdline_packages = " ".join(emerge.cmdline_packages)
     print("Starting fast-emerge.")
     print(" Building package %s on %s" % (cmdline_packages,
-                                          deps.board or "root"))
+                                          deps.sysroot or "root"))
 
   deps_tree, deps_info = deps.GenDependencyTree()